Charged particle analysis includes identifying a chemical constitution of a substance by separating charged particles (e.g., ions) from the substance and analyzing the separated charged particles. Mass spectrometry is a type of charged particle analysis and generally refers to the measurement of the value of a particle's mass or an implicit determination of the value of the particle's mass by measurement of other physical quantities using spectral data. Mass spectrometry involves determining the mass-to-charge ratio of an ionized molecule or component. When the charge of the ionized particle is known, the mass value of the particle can be determined from a spectrum of mass-to-charge values.
Systems for performing mass spectrometry are usually referred to as mass spectrometers. Mass spectrometer systems generally include an ion source, a mass filter or separator and a detector (e.g., Faraday collector or electron multiplier). For example, a sample of molecules or components can be ionized by electron impact in the ion source to create charged particles. Types of ion sources include, for example, electron-impact, electrospray, microwave, proton transfer reaction, plasma, and/or chemical ionization reaction. Charged particles having different mass values are separated by the mass analyzer into a mass spectrum, for example, by controlled application of electrical or magnetic fields to the charged particles in the filter or separator. Parameters and properties of the filter can determine or select a set of charged particles to be transmitted. For example, the properties of the filter can be such that only particles with a particular mass range traverse the filter to the analyzer. The detector collects the charged particles and communicates with a controller to generate a mass spectrum. The mass spectrum may be displayed, viewed and/or recorded. The relative abundance of mass values in the spectrum is used to determine the composition of the sample (or draw conclusions about the sample) and the mass values or identities of molecules or components of the sample.
A quadrupole mass spectrometer is a type of mass spectrometer that includes a quadrupole mass filter to separate the charged particles based on a mass-to-charge ratio of the charged particles. Quadrupole mass spectrometers are typically designed for known charged particles and known angular acceptance of in-bound charged particles. For high-intensity charged particle sources with solid or liquid sample species, unwanted photons, such as visible-light or x-ray photons and unstable neutral molecules, can be generated by the ion source. These unwanted photons and neutrals can produce or result in error (e.g., noise) in the output spectrum. In particular, when the detector has a sightline to the ion source, unwanted noise can result from these unwanted photons.
Current attempts to reduce signal error by preventing a sightline between an ion source and a detector or between an analyzer and a detector involve, for example, mounting the detector off-axis relative to the source or analyzer, inserting a baffle or photon stop (e.g., Bessel box) between the detector and the source or analyzer and/or employing a second filter to filter out the unwanted photons. These current systems typically require large quadrupole geometries (e.g., 9 mm or larger rod diameters). Additionally, these attempts typically involve additional field-generating elements or deflector structures to provide electric fields transverse to the direction of the ion beam to divert the ion beams off-axis. The additional field-generating elements can adversely impact ion transmission and/or require specifically-tuned energies, leading to the possibility of errors. The field-generating elements can also adversely affect the ion flow into the detector, resulting in ions entering the detector on a path that is not collimated or aligned with the detector structures. This misalignment results in reduced detection of ions that pass into the detector. Some beam diversion systems operate at high voltages, which can increase cost and risk. Many existing mass spectrometer systems operate at relatively coarse vacuum pressures, typically up to 0.1 Pa. Therefore, in such systems it is desirable to avoid using high voltages due to the risk of electrical arcing and/or general operational safety considerations.
When quadrupole mass spectrometers are used for Residual Gas Analysis (RGA) or analysis of gases which are sampled from a higher pressure into a vacuum chamber, measurements are typically taken over a wide range of pressures, and a primary gas species can change. For RGA, small quadrupole geometries (e.g., 6 mm or less) are typically used with a sightline of charged particle flow from the charged particle source to the quadrupole filter or lens. With a sightline of charged particle flow from the charged particle source to the quadrupole filter, a baseline signal of an output spectrum can change with changes of species and/or pressure. Thus, the baseline of the output spectrum may obscure the true output spectrum.